Apparatus for burning materials of cement and the like

ABSTRACT

An apparatus for burning the materials of cement and the like is disclosed wherein a calcining furnace is constructed so as to attain the optimum thermal efficiency, and an air inlet provided with means for selectively opening or closing the inlet is attached to a duct for conveying combustion air from a cooling device to the calcining furnace so that the introduction of the combustion air may be controlled.

7/1944 Ruthruff 34/57 R United States Patent 1191 '11 11 3,869,248

Hirai et a1. Mar. 4, 1975 1 APPARATUS FOR BURNING MATERIALS 2,580,235 12/1951 Le11ep 432/61 OF CEMENT AND THE LIKE 3,212,764 10/1965 Muller et a1. 432/58 3,452,968 7/1969 Shimizu et a1]. 432/58 Inventors: Yoshio HlraI; Yoshlml Yamamoto, 3,544,093 12/1970 Fisher 432/176 both of Tokyo, Japan 3,664,650 5/1972 Weber et al. 34/57 R [73] Assignee: Ishikawajima-Harima Jukogyo Kabushiki Kaisha, Tokyo-to, Japan Primary E.\'am1nerJohn J. Camby 1 Flledl y 8, 1973 Assistant Examiner-Henry C. Yuen [21] Appl 361,437 Attorney, Agent, or FirmScrive1ner Parker Scrivener & Clarke [30] Foreign Application Priority Data May 20, 1972 Japan 47-58985 Oct. 4, 1972 Japan 47-99552 Jan. 8, 1973 Japan 48-5032 [57] ABSTRACT 521 US. Cl 432/106, 432/58, 34/57 R An apparatus for burning the materials of cement and [51] Int. Cl. F27b 7/02 the like is disclosed wherein a calcining furnace is 58 1 Field of Search 432/103-107, constructed so as to attain the Optimum thermal effi- 432 14 1 1 27 5 32 117 133 ciency, and an air inlet provided with means for selec- 129430; 34/57 95 10, 103 113415 tively opening or closing the inlet is attached to a duct 123430 140 141; 431/173 for conveying combustion air from a cooling device to the calcining furnace so that the introduction of the 5 References Cited combustion air may be controlled.

UNlTED STATES PATENTS 2,352,738 2 Claims, 7 Drawing Figures APPARATUS FOR BURNING MATERIALS OF CEMENT AND THE LIKE The so-called suspension preheater type rotary kilns have been widely used for burning finely ground particles such as materials for cement because of their high efficiency and large capacity. However since the total thermal load is applied to a rotary kiln, the thermal efficiency as Well as the capacity are limited, and the service life of refractory brick used in the burning zone is remarkably reduced so that the cost and labor for replacing the refractory brick are increased. In order to overcome these problems, there has been proposed a burning process employing a calcining furnace. In the conventional process not employing a calcining furnace the calcination (endothermic reaction) of lime stone, which is one of thematerials for manufacture of cement clinker, is carried out only by 40 percent in the preheater whereas the remaining calcining reaction and the burning reaction (exothermic reaction) are carried out in the kiln with a result of low efficiency. However according to the burning process employing a calcining furnace only a small fraction of the calcining and burning reactions is carried out in the rotary kiln so that the rotary kiln may be reduced in size.

Next referring to the accompaning drawing, the conventional burning apparatus will be described. FIGS. 1(A), (B), and (C) illustrate calcining furnaces employed in the conventional burning apparatus; FIG. 2 is a schematic view illustrating a conventional burning apparatus of the type employing a calcining furnace; and FIG. 3 is a schematic view illustrating a conventional burning apparatus not employing a calcining furnace. Throughout the figures same reference numerals are used to designate similar parts.

For burning. the cement materials the ideal method is to transfer the heat evolved by the combustion of fuel to the finely ground materials suspended in the air flow because the highest thermal efficiency may be attained.

For this purpose in the calcining furnace shown in FIG. 1(A), an air inlet duct b is communicated to the bottom of the main body a of the calcining furnace, and a burner c and a feed inlet d are provided at the top so that both the feed and fuel may be simultaneously charged into the furnace downwardly. However this calcining furnace has a defect that the finely ground particles tend to be mixed into the burning zones of the fuel particles so that when the temperature of the charge is low the combustion is completely stopped.

In the calcining furnace of the type shown in FIG.

1(B), the air inlet duct b is tagentially disposed with respect to the main body a, and the feed inlet d as well as the burner c are disposed within the air duct b. This calcining furnace also has a defect that the wear of the refractory materials is extremely increased because the fuel is injected along the furnace wall and the feed tends to adhere to the furnace wall. a In the calcining furnace of the type shown in FIG. 1(C), the air inlet duct b is connected to the bottom of the main body a, and the burner c is disposed at the bottom so as to inject the fuel upwardly. The feed inlet d is disposed at the top. The extremely high temperature zone is formed adjacent to the burner because the density of the feed therearound is low, and since the burner c is directed upwardly the maintenance is very difficult.

In the calcining furnaces of the types described above, the finely ground raw meal is exposed in the high-temperature burning zone so that the alkali contents in the raw meal are vaporized, resulting in the adverse effects on the product. That is, when the temperature is higher than 1,100C the alkali contents in the whose formation is more or less influenced by the partial pressure of oxygen, is increased exponentially when the temperature rises in excess of'l ,200C. The temperature of the furnace shown in FIG. 1(B) reaches an extremely high temperature of the order ofl,800C to 2,000C so that the formation of nitrogen oxides cannot be avoided.

As shown in FIG. 2, in the apparatus adapted to carry out the burning method employing a calcining furnace, cyclones e e e and e.,, which are intercommunicated by ducts f f and f3, are arranged as in the case of the conventional suspension preheater and communicated through a kiln inlet chamber g with a rotary kiln h, the lower end of which is connected through a kiln hood i to a cooler j. The raw meal fed into the duct f by a feeder k is heated by the heated gases flowing from the cyclone e and collected in the cyclone e and dropped into the duct f In like manner the raw meal is transferred from the duct f the cyclone e and the duct f to the cyclone 2 so that it is sufficiently preheated before it is chaged into a calcining furnace a. In the calcining furnace almost all of the calcining reaction of the raw meal is accomplished by the heat of combustion of fuel injected through the burner c, and the calcined raw meal is discharged through the duct 1 together with the exhaust gases into the cyclone e From the cyclone e, the raw meal is charged into the rotary kiln h through the kiln inlet chamber g and is burnt by the heat of combustion of fuel injected through a burner m provided in the hood i. The burnt clinker is cooled by the cooler j.

The exhaust gases with a temperature of about 1,lO0C to 1,200C are discharged through the inlet chamber g and a duct n to the duct 1 and mixed with the exhaust gases with a temperature of about 850C discharged from the calcining furnace a. While the mixed exhause gases flow through the cyclone e.,, the duct f;,, the cyclone e the duct f the cyclone 2 the duct f and the cyclone 2 the heat is transferred to the finely ground raw meal, and are finally discharged through an exhaust duct 0 by an exhaust fan p. The hightemperature air which has been heated by the red-hot clinker in the coolerj is introduced through a secondary air duct q into the calcining furnace a.

Next the pressure balance in the apparatus with the above construction will be described. The pressure loss in the kiln system AP;, is generally 20 30 mm A whereas the pressure loss in the calcining furnace system AP is 200 mm A,,, so that a blower r must be provided for the calcining furnace system. The exhaust gases discharged from the cooler j contain a large amount of clinker dust so that a highly efficient dust collector s must be provided in order to prevent the abrasion and wear of the blower r. Therefore the capacity of the blower r must be such that it compensates sufficiently the pressure difference AP AP +300 A,,.

As discribed above in the conventional apparatus of the type described hereinbefore the blower for compensating the pressure loss in the calcining furnace system as well as the dust collector must be provided so that the secondary air temperature is rather limited to 350C 400C at the most, thus resulting in the decrease in thermal efficiency. As a result the fuel cost is increased, and there is a danger that the high temperature gases jet out of the apparatus because the internal pressure in the calcining furnace system is raised to a positive pressure in some portions. Furthermore the power consumption of the blower is remarkably increased. These problems are all attributed to the fact that the blower must be provided in order to raise the pressure in the calcining furnace system.

When the apparatus is started, the burner m is ignited to heat the rotary kiln h and the kiln exhaust gases are introduced into the preheater to heat the same. It takes a long time before the rotary kiln is heated to a desired temperature so that when the exhaust gases are continuously introduced into the preheater, the latter is overheated. In the worst case the blower is burnt out. To overcome this problem the conventional apparatus is provided with an auxiliary chimney or stack at the inlet chamber which is an interface between the preheater and the rotary kiln, so that the exhaust gases may be discharged into the atmosphere until the rotary kiln is sufficiently heated. Furthermore the auxiliary chimney is further used to discharge the exhaust gases from the rotary kiln not only when the kiln operation is suspended but also when the raw meal supply is suspended or in case of the emergency when the blower is stopped due to the power failure so that the overheating of the preheater may be prevented.

The present apparatus with an auxiliary chimney is shown in FIG. 3. It comprises the cyclones e 2 the ducts f f intercommunicating the cyclones, an auxiliary chimney t, a chimney damper u, and a preheater damper v disposed within a duct w. Furthermore an exhaust damper is provided within the exhaust gas duct intercommunicating between the cyclone e and the blower p. The apparatus shown in FIG. 3 is not provided with a calcining furnace opposed to the apparatus shown in FIG. 2.

When the raw materials remain in the rotary kiln, they are discharged under the pressure of the hot gases into the atmosphere in a large quantity for a relatively long time, thus resulting in the atmospheric pollution. The damper which is used to switch the flow of the exhaust gases to the auxiliary chimney or to the preheater must be made of the heat-resisting and refractory materials and be rigid in construction. Furthermore the damper is complex in construction and its operation is not easy.

To overcome these problems, there has been proposed a method in which an exhaust duct or outlet y provided with a damper x is attached to the preheater unit. The exhaust gas outlet y is generally inserted immediately before the uppermost cyclone 2, so that not only the heating may be facilitated by the draft effect when the kiln burner is ignited but also the cooling air may be sucked through the exhaust outlet y when the fuel quantity is increased, when the apparatus is operated under the normal conditions or when the apparatus must be suspended immediately because of the emergency, thereby preventing the overheating of the uppermost cyclone e which is not lined with refractory materials and of the blower. However when the damper is installed at a portion where the pressure is negative and of the order of -6OO mm Aq. to 800 mm Aq., even a very slight opening or closing of the damper will largely affect the draft in the kiln. Thus the damper control is very difficult and the kiln operation is adversely affected. Furthermore there arises the problem of sealing, and the air leakage occurs even when the damper is closed in the normal operation so that the load of the blower or exhaust fan must be increased. In some cases the overall efficiency of the apparatus is decreased. Moreover the damper is generally installed at an elevated position, about meters from the ground which, of course varies depending upon the capacity of the apparatus, so that when the damper is opened in case of an emergencysuspension of the apparatus a large amount of heated gases in the kiln is discharged by the stack draft effect, thus resulting in the temperature rise of the upper portions which should be normally cooled. Furthermore it is not preferable to install the damper at such an elevated position in view of the maintenance and repair.

One of the objects of the present invention is therefore to maintain the stable combustion in a calcining furnace with a predetermined thermal efficiency.

Another object of the present invention is to eliminate the use of a blower for increasing the pressure in the calcining furnace system and to overcome the ad-- verse thermal effects upon the structual members of the preheating unit.

A further object of the present invention is to remarkably increase the overall thermal efficiency of the ,burning apparatus and to overcome the hazards such as the burst of the high temperature gases.

Briefly stated, according to the present invention a calcining furnace is provided with a plurality of burners which are so arranged that the combustion of fuel within the furnace becomes maximum in the gas flow immediately below a furnace charge inlet and that the velocity of the gas flow within the furnace becomes faster than the flame traveling speed. The calcining furnace is interposed between a suspension preheater and a rotary kiln, and the combustion air consisting of the kiln exhaust gases and the high temperature secondary air is introduced into the calcining furnace. A reduced portion in the form of an orifice is formed within a kiln exhaust gas duct and a control damper is disposed within a secondary air duct so that the introduction of the combustion air into the calcining furnace may be controlled and the use of a blower for increasing the pressure in the secondary air duct may be eliminated. A duct for conveying the air from a cooler in which the clinker discharged from the rotary kiln is cooled, to the calcining furnace is provided with an air inlet which in turn is provided with means such as a damper for selectively opening or closing the air inlet. Thus the above and other objects of the present invention may be accomplished.

The present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawing in which;

FIGS. 4(A) and 4(8) are a top and side views of a calcining furnace in accord with the present invention;

FIG. 5 is a diagrammatic view illustrating a burning apparatus with a calcining furnace in accord with the present invention;

FIG. 6 is a view used for the explanation of the relation between a calcining furnace and a kiln exhaust gas flow path; and

FIG. 7 is a diagrammatic view illustrating another embodiment of a burning apparatus with a calcining furnace in accord with the present invention.

Same reference numerals are used to designate similar parts throughout the figures.

In a calcining furnace shown in FIGS. 4(A) and 4(8) a combustion air inlet duct'6 is connected to the bottom of the furnace 1 whereas an exhaust gas duct 7 is connected to the upper periphery thereof. The raw meal charged into the furnace 1 through a feed inlet 5 disposed at the top thereof flows in the direction indicated by the solid lines, and a plurality of burners (three burners in the instant embodiment) 2,3 and 4 are also disposed at the top of the furnace 1. The capacity of the burners 2,3 and 4 is reduced in the order named so that the combustion ratio or condition may be varied. Alternatively the combustion ratio may be varied by a number of burners ignited.

The velocity of the combustion air introduced from the air duct 6 is so selected as to become sufficiently higher than the flame propagation or traveling speed, and the opposed or counter flow combustion method is employed so that almost no flame is generated and the finely divided fuel particles which are floating together with the finely ground raw meal in the direction indicated by the broken arrows in the furnace 1 are burnt. As a result no extremely high temperature zone is formed, but a uniform low temperature zone (850- 900C) may be formed. Since the velocity of the combustion air is higher than the flame traveling speed, the heat transfer to the finely ground raw meal is largely effected by the forced convection and the heat transfer by radiation is only about 10 percent. Therefore the sufficient heat transfer may be effected even when the furnace is maintaimed at a low temperature. In the instant embodiment the burners have been shown as being disposed at the top of the furnace but it will be understood that the positions of the burners may be changed as the need demands. The optimum positions and capacity of the burners and the combustion ratio by a number of burners may be varied arbitrarily depending upon the construction of the furnace as well as the flow pattern of the raw meal and the fuel therein.

The mode of operation of the first embodiment of a burning apparatus with a calcining furnace in accord with the present invention shown FIGS. 5 and 6 is substantially similar to that of the conventional apparatus shown in FIG. 2. The raw meal fed into a duct 12 through a feeder 19 is sufficiently preheated when they flow through a cyclone 8, a duct 13, a cyclone 9, a duct 14 and a cyclone 10 in the order named before they are charged into the calcining furnace l. The raw meal is almost completely calcined in the furnace 1 by the heat of combustion of the fuel injected through the burners 2,3 and 4 into the furnace, and the calcined raw meal is charged into a rotary kiln 16 through a duct 7, a cyclone 11 and an inlet chamber 15. The raw meal in the kiln 16 is burnt by the heat of combustion of the fuel injected through a burner 20 disposed in a kiln hood l7, and the burnt clinker is cooled by a cooler 18.

red-hot clinker in the cooler 18 is introduced into a dust precipitation chamber 24 so that the dust-free secondary air may be introduced into the calcining furnace 1 through the duct 6 and that a reduced portion 25 in the form of a fixed orifice is formed in a duct 21 for introducing the exhaust gases discharged from the inlet chamber 15 into the secondary air duct 6 and a control damper 26 is disposed in the secondary air duct 6.

Unlike the apparatus shown in FIG. 2, the kiln exhaust gases (about 1,100C l,200C) are directed from the inlet chamber 15 through the duct 21 to the secondary air duct 6 where they are mixed with the secondary air (700C 750C) and forced into the calcining furnace 1. The exhaust gases discharged from the calcining furnace 1 can sufficiently heat the finely ground raw meal when they flow through the cyclone 11, the duct 14, the cyclone 10, the duct 13, the cyclone 9, the duct 12 and the cyclone 8, and are discharged through the exhaust gas duct 22 by a blower 23. The pressure loss in the secondary duct system is substantially equal to the pressure loss AP 20 30 mm Aq. in the kiln system so that the means for compensating the pressure loss in the secondary air duct system may be eliminated. Only the dust precipitation chamber 24 which is very simple in construction is provided, and the pressure loss in the secondary air duct system AP including the pressure loss-in the dust precipitaion chamber 24 is of the order of mm Aq. Therefore the pressure difference is AP, AP 30 mm Aq. In order to compensate the pressure difference, a pressure control damper may be disposed within the exhaust gas duct 21, but the temperature of the exhaust gases of the kiln is extremely high so that the damper may be easily susceptible to damages. In order to overcome this problem according to the present invention the reduced portion 25 is formed in the duct 21 so that the pressure loss AP becomes slightly in excess of the pressure loss AP,, and the control damper 26 is disposed in the secondary air duct 6 through which flows the gases of a relatively low temperature so that the damages to the damper 26 may be minimized. The secondary air is forced into the calcining furnace after it has been mixed with the exhaust gases from the kiln so that there was a fear that the combustion efficiency would be decreased due to the decrease in oxygen content. However the experiments conducted by the inventors proved that when the combustion ratios in the kiln and the calcining furnace are so controlled that the oxygen content of the secondary air at the inlet of the furnace may be maintained in excess of a required quantity (for example 0 higher than 12 15 percent the adverse effect on the combustion in the furnace may be avoided and the desired operations may accomplished.

The second embodiment of the present invention shown in FIG. 7 is substantially similar in construction and operation to that shown in FIGS. 5 and 6 except that a secondary damper 27 is disposed within the secondary air duct 6 connecting the cooler 18 to the calcining furnace l and an air inlet 29 in which is disposed a damper 28 is connected to the secondary air duct 6. An exhaust damper 30 is disposed within the exhaust duct 22, and a blower 31 is connected to the cooler 18 for discharging the excess air therefrom.

In the burning apparatus of the types described hereinbefore, the preheated raw meal is charged into the calcining furnace 1 from the third stage cyclone 10, and the calcined raw meal is introduced into the fourthstage cyclone 11 and collected. The calcined raw meal in the cyclone 11 is charged into the rotary kiln 16 through its inlet chamber for burning. The secondary air to be forced into the calcining furnace 1 is generally the high temperature air heated by the red-hot clinker in the cooler 18 discharged therein from the kiln l6 and mixed with the exhaust gases discharged from the kiln inlet chamber 15 in the secondary air duct 6.

Next the mode of operation of the inlet 29 attached to the secondary air duct 6 will be described in detail hereinafter. When the kiln 16 is fired, the damper 28 in the inlet 29 is normally closed and the main exhaust fan 23 is driven so that the cooled air may be sucked from the cooler 18 through the secondary air duct when the secondary air duct damper 27 is opened. Therefore the preheating unit may be prevented from being heated in excess of a certain temperature. After the temperature of the rotary kiln 16 has been sufficiently raised, the calcining furnace 1 is ignited and then the raw meal is fed through the feeder 19 so that the operation may be started in a simple manner. When the raw materials remain in the rotary kiln 16 when the operation is started or stopped, the red hot clinker is discharged into the cooler so that the high temperature air may be extracted therefrom. In this case, the inlet damper 28 is opened so that the secondary air may be cooled thereby preventing the overheat of the preheating unit. Furthermore, in the case of kiln shut-down, the sufficient amount of air may be supplied to the preheating unit through the inlet 29 so that the precipitation and accumulation of the suspended finely ground raw meal may be prevented. The excess air in the cooler may be discharged by the exhaust fan 31 after the dust is removed therefrom. In case of the emergency suspension of the apparatus, the cool air may be sucked through the inlet 29 so as to prevent the preheating unit from being overheated. In case of the breakdown of the main exhaust fan 23, the damper 30 in the exhaust duct 22 is closed thereby suspending the gas flow in the apparatus to prevent the overheating.

It will be understood that the present invention is not limited to the above embodiments and that various modifications and variations can be effected without departing the true spirit of the present invention.

As described hereinbefore the calcining furnace of the type described with reference to FIGS. 4 and 5 is used so that no high temperature zone is formed in the furnace. As a result the vaporization of the alkali content of the raw meal may be prevented, thus eliminating the adverse effects caused by the evaporation of the alkali content. The combustion with a less excess air may be attained and a low temperature atmosphere (about 850C 900C) may be formed so that the formation of nitrogen oxides may be eliminated, thus eliminating the cause of the atmospheric pollution. Since the temperature in the furnace may be maintained at a relatively low temperature, it will not be required to use the refractory materials adapted to resist against extremely high temperature for the construction of the furnace wall. Furthermore the service life of the furnace wall may be increased. Since a plurality of burners are installed, the re-ignition is possible so that the complete shutdown of the furnace may be prevented. Moreover due to the dispersion combustion the thermal efficiency may be increased and the combustion with a less excess air may become possible. Thus the overall cost of fuel may be reduced.

In the first embodiment of the present invention, the use of the blower for increasing the pressure of the secondary air for the calcining furnace may be eliminated so that the secondary air may be heated to a sufficiently high temperature, thus resulting in the reduction in fuel cost. All of the pressures in the burning apparatus are negative so that the burst of the high temperature gases may be prevented, and the heat of the exhaust gases from the rotary kiln may be effectively recovered thus resulting in the further reduction of fuel cost. The gases introduced into the cyclone system may be relatively lowered in temperature so that the problems such as the adhesion of coating may be overcome. The temperature of the exhaust gases from the burning apparatus may be also lowered, thus resulting in the increase in thermal efficiency.

In the second embodiment shown in FIG. 7, the inlet provided with the damper is attached to the secondary air duct so that the auxiliary chimney attached to the conventional burning apparatus maybe eliminated and the exhaust gases may be prevented from being always discharged into the surrounding atmosphere. Thus the pollution problem may be completely overcome. Furthermore since the negative pressure in the secondary air duct 6 is always of the order of 10 to 50 mm Aq. the sealing of the inlet damper may be simplified and the quantity of leaking air may be minimized. The effect on the kiln draft due to the damper adjustment may be also minimized so that the gas temperatures at various portions may be controlled in an ideal manner without adversely affecting the operation. As to the construction the component parts are installed at a relatively low position so that the draft of the heated gases from the kiln may be prevented. Therefore only the preheating unit may be cooled without cooling the kiln. (It is unpreferable to cool kiln too rapidly 'in view of the protection of refractory brick). The inlet 29 may be connected to any portion of the secondary air duct 6 without adversely affecting the operation. For example the inlet may be provided adjacent to the kiln hood where the operators are always working so that not only the inspection, maintenance and repair may be facilitated but also the manual operation of the damper 28 may be ensured in case of the emergency when the remote control system is out of order. Since the inlet is provided the preheating unit may be always maintained below a predetermined temperature even when the fuel supply is increased, and the cool air may be mixed with the high temperature secondary air or exhaust gases so that the rapid cooling of the preheating unit may be prevented. As a result the adverse thermal effects on the structure members thereof due to the rapid cooling may be eliminated.

What is claimed is:

l. A suspension type rotary kiln apparatus for burning the materials of cement, comprising a. a calcinating furnace (1) having at its bottom a combustion air inlet and at its top a feed inlet (5) and an exhaust gas outlet, said furnace including also a plurality of burners (2, 3, 4) for establishing a given burning ratio;

b. suspension preheater means including a plurality of cyclones (8, 9, 10) for feeding preheated raw meal to the feed inlet of said furnace;

c. a cyclone (11) having an inlet connected with the exhaust gas outlet of said furnace, said cyclone having an exhaust gas outlet connected with the lower portion of said suspension preheater means, and a raw meal outlet;

d. exhaust fan means (23) connected with the upper portion of said suspension preheater means for drawing exhaust gases therethrough;

e. rotary kiln means (16) having at one end an inlet chamber provided with an inlet connected with the raw meal outlet of said cyclone, said inlet chamber including also an exhaust gas outlet, said rotary kiln means including at its other end an outlet;

. cooler means (18) having an inlet connected with the outlet of said kiln means for receiving the burnt materials produced thereby, said cooler means including also a combustion air outlet;

g. a secondary air duct (6) connecting the combustion air outlet of said cooler means with the combustion air inlet of said calcinating furnace;

h. a branch duct (21) containing a reduced portion defining an orifice (25) for connecting the exhaust gas outlet of said inlet chamber with said secondary air duct adjacent said furnace, whereby the high temperature exhaust gas from the kiln is mixed with the high temperature combustion air from the cooler; and

. damper means (26, 2 7) in said secondary air duct intermediate said cooler and said branch duct for regulating the quantity of the combustion air from the cooler that is mixed with the high temperature exhaust gases from the kiln and is supplied to the furnace, said damper means being so adjusted and the restriction of the orifice in said branch duct being so dimensioned that the pressure loss in the secondary duct system substantially equals the pressure loss in the kiln system, the capacity of said exhaust fan means being so selected relative to the burning ratio of the burners that the velocity of the combustion air introduced into the furnace from the secondary air duct is higher than the flame propagation speed, whereby almost no flame is generated and the finely divided fuel particles which are floating with the finely ground raw material are burnt at a relatively low temperature and without the formation of a high temperature zone in the furnace.

2. Apparatus as defined in claim 1, and further wherein said secondary air duct contains an air inlet opening (29.) intermediate said damper means and said cooler means, and an auxiliary damper (28) for regulating the amount of air supplied to said secondary air duct via said inlet opening. 

1. A suspension type rotary kiln apparatus for burning the materials of cement, comprising a. a calcinating furnace (1) having at its bottom a combustion air inlet and at its top a feed inlet (5) and an exhaust gas outlet, said furnace including also a plurality of burners (2, 3, 4) for establishing a given burning ratio; b. suspension preheater means including a plurality of cyclones (8, 9, 10) for feeding preheated raw meal to the feed inlet of said furnace; c. a cyclone (11) having an inlet connected with the exhaust gas outlet of said furnace, said cyclone having an exhaust gas outlet connected with the lower portion of said suspension preheater means, and a raw meal outlet; d. exhaust fan means (23) connected with the upper portion of said suspension preheater means for drawing exhaust gases therethrough; e. rotary kiln means (16) having at one end an inlet chamber (15) provided with an inlet connected with the raw meal outlet of said cyclone, said inlet chamber including also an exhaust gas outlet, said rotary kiln means including at its other end an outlet; f. cooler means (18) having an inlet connected with the outlet of said kiln means for receiving the burnt materials produced thereby, said cooler means including also a combustion air outlet; g. a secondary air duct (6) connecting the combustion air outlet of said cooler means with the combustion air inlet of said calcinating furnace; h. a branch duct (21) containing a reduced portion defining an orifice (25) for connecting the exhaust gas outlet of said inlet chamber with said secondary air duct adjacent said furnace, whereby the high temperature exhaust gas from the kiln is mixed with the high temperature combustion air from the cooler; and i. damper means (26, 27) in said secondary air duct intermediate said cooler and said branch duct for regulating the quantity of the combustion air from the cooler that is mixed with the high temperature exhaust gases from the kiln and is supplied to the furnace, said damper means being so adjusted and the restriction of the orifice in said branch duct being so dimensioned that tHe pressure loss in the secondary duct system substantially equals the pressure loss in the kiln system, the capacity of said exhaust fan means being so selected relative to the burning ratio of the burners that the velocity of the combustion air introduced into the furnace from the secondary air duct is higher than the flame propagation speed, whereby almost no flame is generated and the finely divided fuel particles which are floating with the finely ground raw material are burnt at a relatively low temperature and without the formation of a high temperature zone in the furnace.
 2. Apparatus as defined in claim 1, and further wherein said secondary air duct contains an air inlet opening (29) intermediate said damper means and said cooler means, and an auxiliary damper (28) for regulating the amount of air supplied to said secondary air duct via said inlet opening. 